The present invention relates to subterranean fluids, and more particularly, to subterranean fluids comprising a hydraulic cement in an invert emulsion, and methods of using such fluids in subterranean operations.
During the drilling of a well bore in a subterranean formation, a drilling fluid may be circulated through a drill pipe and drill bit into the well bore, and subsequently flow upward through the well bore to the surface. The drilling fluid functions, inter alia, to cool the drill bit, lubricate the rotating drill pipe to prevent it from sticking to the walls of the well bore, prevent blowouts by providing hydrostatic pressure to counteract the sudden entrance into the well bore of high pressure formation fluids, and remove drilled cuttings from the well bore. While drilling fluids are not settable, e.g., they do not set into hard impermeable sealing masses when static, drilling fluids may increase in gel strength over time. Typically, after a well bore is drilled to a desired final depth, the drill pipe and drill bit are withdrawn from the well bore and the drilling fluid is left therein so as to, inter alia, provide hydrostatic pressure on permeable formations penetrated by the well bore, thereby preventing the flow of formation fluids into the well bore.
A common subsequent step in completing the well bore usually involves placing a pipe string, e.g., casing, into the well bore. Depending upon factors such as, inter alia, the depth of the well bore and any difficulties in placing the pipe string therein, the drilling fluid may remain relatively static in the well bore for an extended period of time, e.g., up to 2 weeks. During that time, the drilling fluid may progressively increase in gel strength whereby portions of the drilling fluid in the well bore may become increasingly difficult to displace.
Upon placement of the pipe string in the well bore, primary cementing is typically performed whereby the pipe string disposed in the well bore is cemented by pumping a cement composition through the pipe string and into an annulus between the pipe string and the walls of the well bore, thereby displacing the drilling fluid in the annulus. However, if the drilling fluid has developed sufficient gel strength during its residence within the well bore, an operator may be unable to displace all of the drilling fluid with the cement composition. Accordingly, portions of the drilling fluid in the well bore may be bypassed by the cement composition. This is problematic because the drilling fluid is generally not settable; therefore, formation fluids may enter and flow along the well bore, which is highly undesirable.
Operators have attempted to solve this problem by developing settable spotting fluid compositions, inter alia, to displace drilling fluids from well bores promptly after their use. However, these methods have not met with success, as conventional settable spotting fluids include blast furnace slag and other hydraulic components which may begin to set at relatively low temperatures, e.g., temperatures less than about 90° F. Also, certain slag-containing settable spotting fluids may be intolerant to cement composition contamination, causing the settable spotting fluids to cement prematurely upon contact with well cement. Counteracting this tendency to prematurely set, oftentimes a strong set retarder will be added to the spotting fluid, and the spotting fluid is often separated from the cement composition by a spacer fluid.
Conventional settable spotting fluids may also demonstrate other problems, including but not limited to undesirable instability as well as a general inability to develop significant compressive strength upon setting. For example, the invert emulsions found in most conventional settable spotting fluids typically become unstable within about one week after their formulation. This is problematic because it often necessitates formulating the invert emulsion shortly before placing the spotting fluid in a subterranean formation. If an excessive amount of the invert emulsion is formed, it generally cannot be re-used and is often disposed of, which disposal may further increase the cost of a particular job. Furthermore, the general inability of most conventional settable spotting fluids to develop significant compressive strength upon setting is also problematic, because, inter alia, where formation fluids are present under a pressure sufficient to overcome the settable spotting fluid's low compressive strength, such formation fluids may continue to enter and flow along the well bore, which is undesirable and defeats the major purpose of using settable spotting fluids—to provide zonal isolation.